1. Technical Field
The invention relates to the field of touch selection pads having a matrix of tactile sensors, and, more particularly, to one containing a composite material having an electric conductivity which varies in response to application of pressure in a predetermined director.
2. Description of The Prior Art
Touch selection pads are known from the field of design of telephone station apparatus, household appliances computers and the like which comprise a matrix of individual tactile sensors having varying electrical conductivity when touched. Materials with pressure-dependent electrical resistance are known, the electric conductivity of which inoreases with increasing tactile pressure. At least one such known material contains an electrically conductive plastic, such as silicone rubber, with embedded electrically conducting particles in finely distributed form, that can be used to form the tactile sensors. The dimension of the conducting particles is generally about 0.15 to 0.2 mm and they fill about 15 to 20 percent of the total volume of the material. The pressure-dependent electric conductivity is due to the formation of current paths when the material is touched resulting in a corresponding increase in the number of metallic contacts at the pressure points. In an untouched state, this material has a relatively high resistance and for all practical purposes acts as an insulator.
A known matrix comprises sensors formed of this composite material with an average spacing between the sensors of about 1 mm. This matrix comprises an electrical conductor terminal with pairs of terminals each terminating a pair of electric wires. The known matrix is further provided with a plastic covering and a perforated plate containing a hole for each sensor in which a contact pin is arranged for each hole. The design of this touch pad is therefore relatively complicated. In addition, this touch pad having purely resistive evaluation requires a decoupling diode for each sensor for alleviating noise and crosstalk.
In another known tactile sensor matrix, a foil of electrically conductive rubber is provided, the electric resistance of which decreases with increasing pressure. This rubber foil with a thickness of about 24 um is provided on one flat side with parallel-arranged row electrodes and on its opposite flat side with parallel-arranged column electrodes and between which electrodes strip-shaped spacers are provided. The crossings of the row and column electrodes form a matrix of sensors. Row and column conductors are associated with the electrodes for sensing the status of the matrix crosspoints. The pressure dependence of the material, however, varies over the entire surface of the foil. The measured resistance values are therefore not accurately reproducible for a particular pressure load, and a correspondingly large scattering of the measured values is thus obtained. To compensate for this variation in signal levels depending on the particular selection of rows and columns, a relatively costly electronic circuit is therefore necessary.
Consequently, the touch selection pads of the prior art tend to be complicated by additional electronic circuitry to distinguish true touches or, if the touch pad arrangements are simplified, they tend to experience noise or crosstalk problems.